Polycrystalline silicon and process and apparatus for producing the same

ABSTRACT

Foamed polycrystalline silicon having bubbles therein and an apparent density of 2.20 g/cm 3  or less. This silicon generates an extremely small amount of fine grains by crushing and can be easily crushed. There is also provided a method of producing foamed polycrystalline silicon. There is further provided a polycrystalline silicon production apparatus in which the deposition and melting of silicon are carried out on the inner surface of a cylindrical vessel, a chlorosilane feed pipe is inserted into the cylindrical vessel to a silicon molten liquid, and seal gas is supplied into a space between the cylindrical vessel and the chlorosilane feed pipe.

FIELD OF THE INVENTION

[0001] The present invention relates to novel polycrystalline silicon,and a process and apparatus for producing the same. More specifically,it relates to foamed polycrystalline silicon which is soft and generatesan extremely small amount of fine grains when it is crushed, a method ofproducing the same stably, and a polycrystalline silicon productionapparatus which is suitable for the production of the polycrystallinesilicon, capable of producing polycrystalline silicon continuously andstably at a high speed for a long time and extremely useful from anindustrial point of view.

PRIOR ART

[0002] Heretofore, there have been known various methods of producingpolycrystalline silicon used as a raw material for a semiconductor orphoto voltaic and some of them have already been carried out on anindustrial scale.

[0003] For example, one of the methods is called “Siemens method” that asilicon rod heated at the deposition temperature of silicon byenergization is placed in a bell jar and trichlorosilane (SiHCl₃, to beabbreviated as TCS hereinafter) or monosilane (SiH₄) is contacted to thesilicon rod together with reducing gas such as hydrogen to depositsilicon.

[0004] Demand for granular polycrystalline silicon obtained by crushingthe above polycrystalline silicon to a diameter of about 300 μm to 2 mmis growing. For example, the granular polycrystalline silicon is moltenfor use in semiconductors and photo voltaic.

[0005] There is also known a technology for producing fine granularsilica having a diameter of about 1 μm by introducing the granularpolycrystalline silicon into oxyhydrogen flames to be molten andevaporated.

[0006] Further, a silicon nano-grain which attracts much attention as avisible light emission element is produced by irradiating a silicontarget with an excimer laser beam in a helium atmosphere. If thegranular polycrystalline silicon can be easily acquired as a silicontarget material, the silicon nano-grain can be produced efficiently.

[0007] The above granular polycrystalline silicon has been produced bycrushing a nugget obtained by breaking a silicon rod produced by theSiemens method to a size as big as a fist.

[0008] However, when the granular polycrystalline silicon is to beobtained by breaking the above silicon rod, as breaking is difficult,broken pieces called flaky, needle-like and powdery “fine grains” aregenerated by breaking in large quantities. The fine grains are thesource of dust and difficult to handle. Since there is a possibilitythat fine grains having a diameter of 150 μm or less in particular catchfire, they are discarded carefully. Therefore, the fine grains not onlyreduce yield from the raw material but also require a great deal oflabor for disposal.

[0009] Meanwhile, the above Siemens method is characterized by obtaininghigh-purity silicon and has been carried out as the most general method.However, as silicon is deposited in a batch manner, very troublesomeoperations such as the installation of a silicon rod as a seed, heatingof the silicon rod by energization, deposition, cooling, extraction andcleaning of a bell jar must be carried out.

[0010] Another method for obtaining polycrystalline silicon is adeposition method making use of a fluidized bed. In this method, thefluidized bed is used and the above monosilane is supplied while a smallsilicon seed as big as about 100 μm is supplied as a deposition nucleusto deposit silicon on the silicon seed and extract a silicon grain asbig as 1 to 2 mm continuously.

[0011] This method eliminates the need of terminating a reaction forextracting silicon and makes possible relatively long-term continuousoperation.

[0012] However, in the above method which is carried out on anindustrial scale, as monosilane which has a low deposition temperatureis used as a silicon source material, fine powdery silicon is generatedby the thermal decomposition of the monosilane or silicon is readilydeposited on the wall of a reactor even at a relatively low temperaturerange, thereby making it necessary to clean or exchange the reactorregularly.

[0013] Further, since silicon seeds in a fluidized state to be depositedare violently contacted to the wall of the reactor for a long time andrubbed, the above method also involves a problem with the purity of theformed silicon.

[0014] To solve the above problems of the existing technology, JP-A59-121109, JP-A 54-124896 and JP-A 56-63813 (the term “JP-A” as usedherein means an “unexamined published Japanese patent application”)propose a method in which a reactor is heated at a temperature equal toor higher than the melting point of silicon, a silane is supplied intothe reactor as a source material to be deposited, silicon is depositedand molten, its molten liquid is stored, and silicon in a molten stateor its molten product is solidified by cooling and extracted to theoutside of the reactor continuously or intermittently.

[0015] However, particularly in the method using monosilane, asmonosilane has the property of decomposing by itself even in anatmosphere of relatively low temperature gas and generating fine powderysilicon, blocking in a gas down-stream region is apprehended.

[0016] In any one of the methods conventionally proposed, a connectionportion between the reactor and a silane feed pipe or a portiontherearound has a temperature gradient from the melting temperature to atemperature at which silicon does not deposit. As a result, there isalways a portion having a temperature range at which silicon deposits byself-decomposition and the portion may be blocked with silicon which isdeposited by carrying out the method on an industrial scale.

[0017] No report on simple and effective means of preventing blockingcaused by the deposition of silicon has been made yet.

[0018] JP-A 11-314996 discloses a method of producing crystallinesilicon, for example, polycrystalline silicon, using an apparatuscomprising a heat generating solid, a high-frequency induction coilarranged opposite to the under surface of the heat generating solid andat least one gas outlet formed in the coil, the method comprisingblowing a raw material gas containing a deposition component against theunder surface of the above heat generating solid high-frequencyinduction coil heated by the high-frequency induction coil from theabove gas outlet, depositing and melting the above deposition componenton the under surface of the above heat generating solid, and dropping orflowing down the deposited molten liquid from the bottom of the aboveheat generating solid.

[0019] However, this method has such a problem as high energyconsumption because the high-frequency induction coil which needs to becooled by water to retain its function absorbs heat because thehigh-frequency induction coil is in close vicinity to the heatgenerating solid. This publication is silent about the production offoamed polycrystalline silicon.

OBJECTS OF THE INVENTION

[0020] It is a first object of the present invention to provide foamedpolycrystalline silicon which generates an extremely small amount offine grains by crushing for the production of a crushed product ofpolycrystalline silicon.

[0021] It is a second object of the present invention to provide amethod of producing the above polycrystalline silicon with highreproducibility and stability.

[0022] It is a third object of the present invention to provide anapparatus for producing polycrystalline silicon, which is suitable foruse in the above method of producing polycrystalline silicon, capable ofproducing polycrystalline silicon continuously and stably at a highspeed for a long time and extremely useful from an industrial point ofview.

[0023] Other objects and advantages of the present invention will becomeapparent from the following description.

SUMMARY OF THE INVENTION

[0024] To attain the first object, the inventors of the presentinvention have confirmed that fine grain generation mechanism in thecrushing of polycrystalline silicon is based on the cleavage ofpolycrystalline silicon. That is, since polycrystalline silicon cleaveseasily, when a nugget obtained by breaking the above silicon rod isfurther crushed to obtain granular polycrystalline silicon, flaky andneedle-like fine grains are easily generated in large quantities.

[0025] Based on knowledge that the generation of fine grains by crushingcan be suppressed by providing a structure that polycrystalline siliconis enough to be crushed under much lower stress than necessary stressfor indicating cleavage to a polycrystalline silicon structure, byemploying a bubble enveloping structure which is unknown in the priorart as a form of polycrystalline silicon, energy for crushing siliconcan be caused to act as energy for breaking the wall of a bubble beforeit is applied to the cleaved surface of crystals, thereby making itpossible to drastically reduce the proportion of fine grains to bediscarded compared with ordinary silicon crushed products.

[0026] In order to fully develop an effect obtained by the existence ofthe above bubbles, it has been found that it is effective to adjust theamount of the bubbles to a value corresponding to a specific apparentdensity or less. The present invention has thus been accomplished basedon this finding.

[0027] Therefore, according to the present invention, firstly, the aboveobjects and advantages of the present invention are attained by foamedpolycrystalline silicon which contains bubbles therein and has anapparent density of 2.20 g/cm³ or less, based on the above knowledge.

[0028] To attain the second object of the present invention, although itis known that gas rarely dissolves in a molten metal such as a siliconmolten liquid, the inventors of the present invention have found thatwhen the gas is hydrogen, it can be dissolved in a certain amount. Basedon this knowledge, they have conducted studies and have found that afterhydrogen is contacted to a silicon molten liquid to be dissolved in theliquid, the molten liquid is naturally dropped as droplets andsolidified under specific cooling conditions to obtain solidifiedpolycrystalline silicon containing hydrogen existent in the droplets asbubbles.

[0029] Therefore, according to the present invention, secondly, theabove objects and advantages of the present invention are attained by amethod of producing foamed polycrystalline silicon, comprising naturallydropping droplets of silicon containing hydrogen which is molten in thepresence of hydrogen in 0.2 to 3 seconds and cooling the droplets untilhydrogen bubbles are locked up in the droplets.

[0030] To attain the third object of the present invention, theinventors of the present invention have confirmed that low heatconsumption can be obtained when a heater having a silicon depositionsurface is made cylindrical and the deposition and melting of siliconare carried out on the inner surface of the heater. They have found thefollowing. That is, based on the principles that silicon will notdeposit if a raw material gas is not existent in an area heated to thedeposition temperature of silicon and that silicon will not depositsubstantially if the region where the raw material gas is existent doesnot reach the deposition temperature, it is possible to continuouslyextract silicon in a molten state while the formation of solid siliconon the inner wall of a reactor is extremely effectively suppressed, byusing a chlorosilane whose silicon deposition start temperature iscloser to the melting point of silicon than monosilane as a sourcematerial gas, making the feed pipe of the source material gas open in acylindrical heater as the above heater to directly supply the sourcematerial gas into a high-temperature region for carrying out thedeposition and melting of silicon, supplying hydrogen into the regionand supplying seal gas into the space between the source material gasfeed pipe and the cylindrical heater.

[0031] Therefore, according to the present invention, thirdly, the aboveobjects and advantages of the present invention are attained by apolycrystalline silicon production apparatus comprising:

[0032] (a) a cylindrical vessel having an opening which is a silicontake-out port at the lower end;

[0033] (b) a heater for heating the inner wall from the lower end to adesired height of the cylindrical vessel at a temperature equal to orhigher than the melting point of silicon;

[0034] (c) a chlorosilane feed pipe which is composed of an inner pipehaving a smaller outer diameter than the inner diameter of the abovecylindrical vessel and constituted such that one opening of the innerpipe faces down in a space surrounded by the inner wall heated at atemperature equal to or higher than the melting point of silicon; and

[0035] (d) a first seal gas feed pipe for supplying seal gas into aspace defined by the inner wall of the cylindrical vessel and the outerwall of the chlorosilane feed pipe.

[0036] The foamed polycrystalline silicon of the present invention canbe obtained efficiently by the above apparatus. That is, as hydrogen isexistent in the silicon deposition and melting area in the aboveapparatus, hydrogen can be contacted to a silicon molten liquid formedon the surface of the cylindrical vessel which is a heater and dissolvedin the liquid, the resulting product is naturally dropped from theperiphery of the opening at the lower end of the cylindrical vessel asdroplets, and the droplets are received on a suitable coolant andcollected, thereby making it possible to produce the above foamedpolycrystalline silicon efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037]FIG. 1 is a schematic diagram of a basic embodiment of thepolycrystalline silicon production apparatus of the present invention;

[0038]FIG. 2 is a schematic diagram of another basic embodiment of thepolycrystalline silicon production apparatus of the present invention;

[0039]FIG. 3 is a schematic diagram of a typical embodiment of thepolycrystalline silicon production apparatus of the present invention;

[0040]FIG. 4 is a schematic diagram of another typical embodiment of thepolycrystalline silicon production apparatus of the present invention;

[0041]FIG. 5 is a sectional view of a typical embodiment of acylindrical vessel used in the polycrystalline silicon productionapparatus of the present invention; and

[0042]FIG. 6 is a sectional view of another typical embodiment of acylindrical vessel used in the polycrystalline silicon productionapparatus of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0043] Bubbles are existent in the interior of the foamedpolycrystalline silicon of the present invention. Thus, apolycrystalline silicon structure containing bubbles therein has beenunknown heretofore and is a big feature of the foamed polycrystallinesilicon of the present invention.

[0044] That is, as for a polycrystalline silicon rod obtained by theabove Siemens method, hydrogen gas is used as a raw material butdeposited polycrystalline silicon is solid and hydrogen does notdissolve in the polycrystalline silicon.

[0045] There is also proposed a method in which silicon is depositedusing hydrogen as one of raw materials and collected as a molten liquid.Since the molten liquid is extracted outside a hydrogen atmosphere andsolidified in the method, hydrogen gas contained in the solid isdiffused and gone in a molten liquid state.

[0046] There is further proposed a method of producing polycrystallinesilicon seed by dropping silicon formed in hydrogen gas onto a rotarydisk in a molten state and scattering it. As the renewal of the surfaceof a silicon droplet occurs violently when the silicon droplet isscattered in this method, the dissolved hydrogen gas escapes and foamedpolycrystalline silicon in which the dissolved hydrogen gas has growninto bubbles cannot be obtained.

[0047] Further, although polycrystalline silicon obtained by usingmonosilane as a source material gas and growing polysilicon grains in afluidized bed contains a relatively large amount of hydrogen, thehydrogen bonded to silicon is existent in the polycrystalline siliconand cannot be existent as bubbles.

[0048] The foamed polycrystalline silicon of the present invention mayhave any shape if it contains bubbles therein. For example, it isgenerally and preferably in the form of an independent grain having nofixed shape. The independent grain has a volume of preferably 0.01 to 3cc, particularly preferably 0.05 to 1 cc. The grain obtained by theproduction method to be described hereinafter may be in the form of apartly fused agglomerate according to a cooling mode. Agglomeration canbe easily separated by releasing a fused portion by slightly crushingthe agglomerate to obtain the above independent grain having no fixedshape easily.

[0049] The foamed polycrystalline silicon of the present invention ispreferably an assembly of independent grains or an agglomerate ofindependent grains.

[0050] Preferably, 50 g or more of independent grains each having aweight of 0.1 to 2 g are contained based on 100 g of the assembly ofindependent grains. More preferably, 80 g or more of independent grainseach having a weight of 0.1 to 2 g are contained based on the samestandard.

[0051] Further, the foamed polycrystalline silicon grain of the presentinvention contains a plurality of independent bubbles which are existentin a center portion of the grain.

[0052] In the present invention, the amount of bubbles in the foamedpolycrystalline silicon corresponds to an apparent density of 2.20 g/cm³or less, preferably 2.0 g/cm³ or less, more preferably 1.8 g/cm³ orless.

[0053] Although the true density of polycrystalline silicon 3 is 2.33g/cm , when polycrystalline silicon contains bubbles, the apparentdensity thereof lowers. Bubbles are contained in the foamedpolycrystalline silicon of the present invention to ensure that theapparent density should become 2.20 g/cm³ or less, thereby making itpossible to greatly suppress the generation of fine grains by crushing.

[0054] In the present invention, the apparent density is a valueobtained from the volume and weight of the grain measured by apicnometer. Deaeration is carried out by a vacuum deaeration method.More specifically, the method described in Powder Engineering Handbook(published by Nikkan Kogyo Shimbun on Feb. 28, 1986) at pp. 51 to 54 maybe used.

[0055] When the foamed polycrystalline silicon of the present inventionis directly supplied into a crucible for the production ofmonocrystalline silicon as silicon to be recharged as it is light, ithas a merit that a spray of a silicon molten liquid is hardly formed inthe crucible and the silicon is useful even in an uncrushed state.

[0056] In the above foamed polycrystalline silicon, a large number ofbubbles maybe uniformly existent as described above, or one or severallarge bubbles may be existent. However, the diameter of each bubble ispreferably 50 μm or more.

[0057] In the present invention, as the foamed polycrystalline siliconhaving an extremely small apparent density may be difficult to beproduced and handled, polycrystalline silicon preferably has an apparentdensity of 1 g/cm or more.

[0058] According to the present invention, there is also provided acrushed product of the foamed polycrystalline silicon of the presentinvention by making use of the above property of the foamedpolycrystalline silicon of the present invention. This crushed productpreferably has an average grain diameter of more than 200 mm and 5 mm orless. The above average grain diameter is obtained using a JIS-Z8801sieve. This crushed product often has a broken section broken at abubble portion of the foamed polycrystalline silicon.

[0059] Gas existent in the bubbles of the polycrystalline silicon of thepresent invention is generally hydrogen gas according to the productionmethod to be described hereinafter but not limited to this.

[0060] The method of crushing the foamed polycrystalline silicon of thepresent invention is not particularly limited and the crushed product ofpolycrystalline silicon can be obtained at a high yield by suppressingthe generation of fine grains in accordance with the crushing methodusing a known crusher such as a jaw crusher or pin mill.

[0061] The polycrystalline silicon production method of the presentinvention is not particularly limited but is preferably carried out byforming a droplet from silicon molten in a hydrogen gas atmospheremaking use of the fact that hydrogen gas is easily dissolved in asilicon molten liquid, naturally dropping the droplet and cooling it tosuch a state that hydrogen bubbles are locked up in the droplet asdescribed as the above method of producing polycrystalline silicon.

[0062] Although melting or molten silicon may be contacted to hydrogengas to obtain molten silicon in the presence of hydrogen in the methodof producing foamed polycrystalline silicon of the present invention,the deposition of silicon from a chlorosilane and the melting of thesilicon are carried out simultaneously in the presence of hydrogen todissolve hydrogen in a silicon molten liquid most efficiently.

[0063] Stated more specifically, a mixed gas of hydrogen gas and achlorosilane is contacted to the surface of a heater heated at atemperature equal to or higher than the melting point of silicon tocarry out the deposition and melting of silicon simultaneously.

[0064] The above chlorosilane is preferably a chlorosilane containinghydrogen in the molecule, such as trichlorosilane or dichlorosilane, tofurther improve the content of hydrogen in the silicon molten liquid.

[0065] The ratio of hydrogen to the above chlorosilane may be a knownratio without restriction but the molar ratio of hydrogen to thechlorosilane is preferably adjusted to 5 to 50 in order to form ahigh-concentration hydrogen atmosphere.

[0066] This silicon molten liquid containing hydrogen is naturallydropped as a droplet and the above hydrogen bubbles are locked up in thedroplet in 0.2 to 3 seconds. The method of locking up the bubbles in thedroplet is not particularly limited but the method in which the dropletis contacted to a coolant having a surface temperature of 1,100° C. orlower, preferably 1,000 or lower, particularly preferably 500° C. orlower is effective and advantageously used in the present invention.

[0067] In the above method, it is important to naturally drop thesilicon molten liquid as a droplet. That is, over saturated hydrogen gasexistent in the silicon molten liquid gathers and grows into bubblesalong the passage of time. If the molten liquid is directly solidified,the bubbles will go up by the influence of gravity and hydrogen gasdissolved in the liquid will be discharged to the outside extremelyeasily.

[0068] In contrast to this, the above silicon molten liquid is naturallydropped to enter a non-gravity state where no floating force isexistent, whereby gasified hydrogen remains in the droplet. This naturaldrop time is preferably 0.2 to 2 seconds.

[0069] In this case, the mechanism that the bubbles remain in thedroplet and collect in the center portion is assumed as follows. Thatis, when the molten liquid is dropped from a base material holding themolten liquid, a droplet has momentum derived from transformation andtries to become globular due to its surface tension immediately, wherebythe momentum derived from transformation changes into rotary angularmomentum and centrifugal force is applied to the interior of the dropletby the above rotary motion without gravity. This centrifugal forcereplaces gravity and floating force serves to direct bubbles of hydrogenexistent in the interior toward the center portion, whereby bubblesgather in the center portion of the droplet.

[0070] The condition for the collection of the bubbles in the centerportion depends upon the rotary angular speed of the droplet and theelapsed time. As for the initial momentum applied to the droplet, therotary momentum and the angular speed increase as the droplet threadslonger when it is separated. That is, as adhesion between the siliconmolten liquid and the base material becomes higher, bubbles contained inthe droplet collect in the center portion faster and remain more easily.When adhesion with the silicon molten liquid is taken intoconsideration, SiO₂ and silicon nitride may be used as the base materialbut SiC having high wettability or a carbon material which has lowinitial wettability but readily forms a silicide to increase itswettability is preferred to exhibit the effect of the present inventionmore markedly.

[0071] In the above method of the present invention, the time elapsedfrom the time when the silicon droplet separates from the heater to thetime when bubbles are locked up in the droplet must be a time duringwhich bubbles can collect in the center portion of the droplet and beretained to such an extent that the above apparent density of thepresent invention can be attained, for example, 0.2 second or more, morepreferably 0.4 second or more, much more preferably 0.6 second or more.

[0072] Since bubbles collected in the center are scattered and escapedto the outside when they are cooled gradually, the above time is 3seconds or less, preferably 2 seconds or less.

[0073] The time from the time when the silicon droplet separates fromthe heater to the time when bubbles are locked up in the droplet ispreferably made slightly longer when silicon nitride having poorwettability is used as the base material than when SiC which canincrease the angular speed sufficiently is used because the angularspeed given to the droplet slightly differs according to the material ofthe heater.

[0074] In the present invention, in the operation of contacting thedroplet to the coolant, the coolant is not particularly limited and maybe solid, liquid or gas.

[0075] As a preferred example of the coolant, a material which does notreact with silicon substantially, such as silicon, copper or molybdenumis used as the coolant and a silicon molten liquid droplet is droppedonto the coolant, or a liquid refrigerant which does not react withsilicon substantially, such as liquid silicon tetrachloride or liquidnitrogen is used as the coolant and a silicon molten liquid droplet isdropped into the coolant.

[0076] Cooling gas generated by spraying the above refrigerant may becontacted to the silicon molten liquid droplet as a refrigerant.

[0077] When the above solid coolant is used, its surface may be cooledby a known cooling method directly or indirectly as required. There is acase where silicon molten liquid droplets are dropped onto the coolantone after another and solidified with the result that foamedpolycrystalline silicon is piled up. In this case, the topmost surfaceof the foamed polycrystalline silicon functions as the coolant. Toabsorb impact when the silicon molten liquid droplets fall on thesurface of the coolant, the surface of the coolant is preferably uneven.For example, grains such as silicon grains are preferably existent onthe surface. In this case, part of the obtained foamed polycrystallinesilicon is preferably used as the silicon grains.

[0078] The apparatus for carrying out the method of the presentinvention is not particularly limited but an apparatus shown as theabove polycrystalline silicon production apparatus is preferred as theapparatus for dropping silicon molten liquid droplets continuously.

[0079]FIG. 1 and FIG. 2 are schematic diagrams of a basic embodiment ofthe above apparatus. That is, the production apparatus shown in FIG. 1and FIG. 2 comprises:

[0080] (a) a cylindrical vessel having an opening which is a silicontake-out port at the lower end;

[0081] (b) a heater for heating the inner wall from the lower end to adesired height of the cylindrical vessel at a temperature equal to orhigher than the melting point of silicon;

[0082] (c) a chlorosilane feed pipe which is composed of an inner pipehaving a smaller outer diameter than the inner diameter of the abovecylindrical vessel and constituted such that one opening of the innerpipe faces down in the space surrounded by the inner wall heated at atemperature equal to or higher than the melting point of silicon;

[0083] (d) a first seal gas feed pipe for supplying seal gas into thespace defined by the inner wall of the cylindrical vessel and the outerwall of the chlorosilane feed pipe; and further optionally

[0084] (e) a hydrogen feed pipe for supplying hydrogen gas into theabove cylindrical vessel.

[0085] The hydrogen feed pipe may be omitted when hydrogen is suppliedfrom the above first seal gas feed pipe.

[0086] In the silicon production apparatus of the present invention, thecylindrical vessel 1 may have an opening 2 as a silicon take-out portfrom which deposited or molten silicon can fall to the outside of thevessel naturally as will be described hereinafter.

[0087] Therefore, the sectional form of the cylindrical vessel 1 may beany form such as a circular or polygonal form. The cylindrical vessel 1may be formed to have a straight barrel with an equal sectional area atany position as shown in FIGS. 1 to 3 to facilitate its production, orthe sectional area of part of the vessel may be made larger than otherpart as shown in FIG. 4 to improve the conversion of a chlorosilane intosilicon (may be simply referred to as “conversion” hereinafter) byextending the residence time of reaction gas.

[0088] Meanwhile, the open state of the opening 2 of the cylindricalvessel 1 may be such that it is straight open as shown in FIG. 1 or acontraction portion may be formed to reduce the diameter graduallytoward the lower end.

[0089] When the opening 2 of the cylindrical vessel 1 may be constitutedsuch that its periphery is horizontal, silicon molten liquid dropletscan be dropped without a problem. However, the opening is preferablyconstituted such that its periphery is inclined as shown in FIG. 5 orits periphery is wavy as shown in FIG. 6, thereby making it possible tomake uniform the diameters of the silicon molten liquid droplets fallingfrom the periphery of the opening 2.

[0090] Further, to make uniform the diameters of the molten silicondroplets regardless of the shape of the periphery of the above opening,the opening is preferably edged by reducing the thickness toward theend.

[0091] Since the above cylindrical vessel 1 is heated at 1,430° C. ormore and the inside of the vessel is contacted to a chlorosilane orsilicon molten liquid, it is desirable to choose a material which canstand the above temperature condition and a substance to be contactedtherewith for the long-term stable production of silicon.

[0092] Examples of the material include individual materials such ascarbon materials including graphite and ceramic materials includingsilicon carbide (SiC), silicon nitride (Si₃N₄), boron nitride (BN) andaluminum nitride (AlN), and composite materials thereof.

[0093] It is particularly preferred for continuous industrial that acarbon material should be used as the base material and at least acontact portion with the silicon molten liquid should be covered withsilicon nitride, boron nitride or silicon carbide to greatly extend theservice life of the cylindrical vessel.

[0094] In the silicon production apparatus of the present invention, theabove cylindrical vessel 1 is provided with a heater 3 for heating thewall of the cylindrical vessel 1 from the lower end to a desired heightat a temperature equal to or higher than the melting point of silicon.The width to be heated at the above temperature, that is, the height ofthe heater 3 from the lower end of the cylindrical vessel 1 may besuitably determined in consideration of the size of the cylindricalvessel and the above heating temperature and further the amount of achlorosilane to be supplied. As the range of the cylindrical vessel tobe heated at a temperature equal to or higher than the melting point ofsilicon by the heater, the length from the lower end is generally 20 to90%, preferably 30 to 80% of the total length of the cylindrical vessel1.

[0095] Any known heating means may be used as the heater 3 if it canheat the inner wall of the cylindrical vessel at a temperature equal toor higher than the melting point of silicon, that is, 1,430° C. or more.

[0096] The heater is, for example, a device for heating the inner wallof the cylindrical vessel by external energy as shown in FIG. 1. Morespecifically, heaters making use of high frequency, heaters making useof a heating wire and heaters making use of infrared radiation may beused.

[0097] Out of these, heaters making use of high frequency are preferredbecause the cylindrical vessel can be heated at an uniform temperaturewhile the shape of the heating coil for radiating high frequency is madesimple.

[0098] In the silicon production apparatus of the present invention, thechlorosilane feed pipe 5 is used to directly supply a chlorosilane Ainto the space 4 surrounded by the inner wall of the cylindrical vessel1 heated at a temperature equal to or higher than the melting point ofsilicon and is open in the space 4 in a downward direction.

[0099] The term “downward” indicating the opening direction of thechlorosilane feed pipe 5 is not limited to a vertical direction only butincludes a case where the chlorosilane feed pipe 5 is open such that thefed chlorosilane is not contacted to the opening again. However, it isthe most preferred that the chlorosilane feed pipe 5 is installed in adirection perpendicular to the plane.

[0100] The chlorosilane supplied from the chlorosilane feed pipe 5 has ahigher thermal decomposition temperature than monosilane which isanother silicon source material. Even if the inside of the pipe isheated in the space 4 of the cylindrical vessel heated at a temperatureequal to or higher than the melting point of silicon, the chlorosilanedoes not decompose violently but cooling is preferably carried out toprevent the deterioration of the feed pipe by heat or the decompositionof the chlorosilane though it is small in quantity.

[0101] Although the cooling means is not particularly limited, a liquidjacket for cooling by forming a flow passage for a refrigerant such aswater or heat medium oil to supply it from D₁ and discharge it from D₂as shown in FIG. 1 or an air cooling jacket (not shown) for cooling acenter nozzle by forming two or more multi-ring nozzles in thechlorosilane feed pipe to supply a chlorosilane from a center portionand purge cooling gas from the outer ring nozzle may be employed.

[0102] As for the temperature for cooling the chlorosilane feed pipe,the chlorosilane feed pipe may be cooled to such an extent that thematerial forming the feed pipe does not deteriorate considerably,generally a temperature lower than the self-decomposition temperature ofthe fed chlorosilane. The chlorosilane feed pipe is preferably cooled to600° C. or less. More specifically, when TCS or silicon tetrachloride(SiCl₄, to be abbreviated as STC hereinafter) is used as a sourcematerial, it is preferably cooled to 800° C. or less, more preferably600° C. or less, the most preferably 300° C. or less.

[0103] The same material as the cylindrical vessel 1, quartz glass, ironand stainless steel may be used as the material of the chlorosilane feedpipe 5.

[0104] In another embodiment of the silicon production apparatus of thepresent invention in which an expanded portion is formed in part of thecylindrical vessel as shown in FIG. 4, the opening of the abovechlorosilane feed pipe is preferably installed in the space of theexpanded portion. Thereby, the opening can be separated from the heatedinner wall and cooling can be carried out easily to prevent thedeposition of silicon on the chlorosilane feed pipe.

[0105] In the present invention, the first seal gas feed pipe 7 isprovided to supply seal gas B into the space defined by the inner wallof the cylindrical vessel existent above the opening of the chlorosilanefeed pipe 5 and the outer wall of the chlorosilane feed pipe. That is,in the present invention, a chlorosilane supplied as a source materialis directly supplied into a high-temperature space where the melting ofsilicon occurs to prevent the deposition of solid silicon by contactinga low-temperature region where silicon can be deposited but not moltenon the inner wall of the cylindrical vessel. However, a similarlow-temperature region is existent in the space formed by the inner wallof the cylindrical vessel and the outer wall of the chlorosilane feedpipe.

[0106] Therefore, in the apparatus of the present invention, thedeposition of solid silicon in the low-temperature region by entry of amixed gas of a chlorosilane and hydrogen can be effectively prevented byproviding the first seal gas feed pipe 7 for supplying seal gas into theabove space to fill seal gas in the space where the abovelow-temperature region is existent.

[0107] In the present invention, the first seal gas feed pipe 7 is notparticularly limited if it is located above the opening of thechlorosilane feed pipe 5 but preferably attached to the wall of thecylindrical vessel where the heater 3 is not existent.

[0108] The seal gas supplied from the first seal gas feed pipe 7 ispreferably gas which does not form silicon and does not exert a badinfluence upon the formation of silicon in the region where thechlorosilane is existent. Specifically, it is preferably an inert gassuch as argon or helium, or hydrogen to be described hereinafter.

[0109] In this case, it will suffice if the seal gas is supplied to suchan extent that a pressure at which the seal gas always fills the spacewhere the above temperature gradient is existent is maintained. In orderto reduce the supply of the seal gas, the shape of the cylindricalvessel 1 or the shape of the outer wall of the chlorosilane feed pipemay be determined to reduce the sectional area of the whole space or thelower portion.

[0110] In the silicon production apparatus of the present invention, thehydrogen feed pipe for supplying hydrogen to be used in a depositionreaction together with the chlorosilane is not particularly limited ifit is open at a position where it can supply hydrogen into the abovespace 4 of the cylindrical vessel 1 independent of the chlorosilane feedpipe 5.

[0111] Therefore, the hydrogen feed pipe is preferably installed at aposition where a reaction between hydrogen and the chlorosilane can beefficiently carried out in consideration of the structure and size ofthe cylindrical vessel 1 constituting the silicon production apparatus.Stated more specifically, in FIG. 1, it is preferred to supply hydrogenC from the first seal gas feed pipe 7 as the seal gas. As shown in FIG.2, the hydrogen feed pipe 8 for supplying hydrogen C may be connected tothe side wall of the cylindrical vessel 1. As a matter of course, theabove two embodiments may be combined.

[0112] As described above, the polycrystalline silicon productionapparatus of the present invention is characterized in that:

[0113] (1) the deposition and melting of silicon are carried out on theinner wall of the cylindrical vessel,

[0114] (2) the chlorosilane feed pipe is inserted into the siliconmelting region in the inside of the cylindrical vessel, and

[0115] (3) seal gas is supplied into the space between the cylindricalvessel and the chlorosilane feed pipe.

[0116] According to the above feature (1), the heat efficiency of aheated surface for carrying out the deposition and melting of siliconcan be greatly increased industrially advantageously.

[0117] Due to a combination of the features (2) and (3), solid siliconcan be completely prevented from remaining deposited without beingmolten in the apparatus.

[0118] In the silicon production apparatus of the present invention,other structures are not particularly limited but a preferred embodimentis given below. For instance, at least the opening at the lower end ofthe cylindrical vessel is preferably covered by a closed vessel 10connected to an exhaust gas discharge pipe 12 to collect exhaust gasgenerated in the cylindrical vessel 1 efficiently and to collect siliconmolten droplets dropping from the opening 2 of the cylindrical vessel 1by solidifying the droplets by cooling without contacting the outsideair. Thereby, high-purity silicon can be industrially obtained.

[0119] A typical embodiment of the above closed vessel 10 is shown inFIG. 3 and FIG. 4. Preferably, the opening 2 which is a silicon take-outport of the cylindrical vessel 1 is covered, a cooling space 15 intowhich a silicon molten liquid can be dropped is formed, and a gasdischarge pipe 12 for taking out exhaust gas is provided.

[0120] The above closed vessel 10 may be disposed such that it coversthe lower end of the cylindrical vessel in such a manner that an endportion of the opening 2 of the cylindrical vessel 1 projects. Forexample, it may be connected to the outer wall of the cylindrical vesselnear the opening. However, since it is very likely that thelow-temperature region where the above solid silicon separates out isexistent on the surface of the closed vessel at a position away from theconnection position, as shown in FIG. 3 and FIG. 4, it is preferablyconnected to the outer wall of an upper portion of the cylindricalvessel away from the high-temperature region including the opening orprovided to cover the entire cylindrical vessel.

[0121] The chlorosilane contained in the gas exhausted from thecylindrical vessel 1 is close to stable gas composition from whichsilicon is not deposited any more and even if silicon is depositedtherefrom, its amount is very small.

[0122] However, to prevent the deposition of solid silicon on the closedvessel 10 as much as possible, as shown in FIG. 3 and FIG. 4, a secondseal gas feed pipe 11 for supplying seal gas E into the space defined bythe outer wall of the cylindrical vessel and the inner wall of theclosed vessel is preferably provided.

[0123] The type and supply of the above seal gas may be determined inthe same manner as when the seal gas is supplied to the first seal gasfeed pipe 7.

[0124] In the above embodiment, the linear speed of the seal gascirculating around the cylindrical vessel 1 is set to at least 0.1 m/s,preferably 0.5 m/s, the most preferably 1 m/s or more to fully developthe effect of the seal gas.

[0125] The material of the closed vessel 10 is suitably selected frommetal materials, ceramic materials and glass materials but the inside ofa collection chamber made from metal is preferably lined with silicon,Teflon or quartz glass to obtain a firm industrial apparatus and collecthigh-purity silicon at the same time.

[0126] The exhaust gas after the reaction in the cylindrical vessel 1 istaken out from the gas exhaust pipe 12 provided in the closed vessel 10.

[0127] The molten silicon dropped from the cylindrical vessel 1 iscooled while it falls in the cooling space 15 of the closed vessel 10 orwhen it contacts a coolant existent on the bottom, stored in the lowerportion of the vessel as solidified silicon 23 and cooled to atemperature at which it is easy to handle. When the above cooling spaceis formed fully long, granulated silicon is obtained and when thecooling space is short, elastically deformed solid silicon is obtainedby drop impact.

[0128] The foamed polycrystalline silicon of the present invention canbe efficiently produced by suitably setting the length of the space 15in which the silicon molten liquid formed in the presence of hydrogen onthe inner wall of the cylindrical vessel is naturally dropped asdroplets and solidified and conditions for cooling the bottom serving asa coolant.

[0129] It is preferred to provide a cooling gas H feed pipe 13 topromote cooling. Not shown in the figure, a solid or liquid coolant maybe existent at the bottom of the closed vessel 10 separately to coolsilicon molten liquid droplets more powerfully as required. Silicon,copper or molibden may be used as the solid coolant. Liquid silicontetrachloride or liquid nitrogen may be used as the liquid coolant.

[0130] A take-out port 17 for continuously or intermittently taking outsolidified silicon 1 may be formed in the closed vessel 10 as required.When silicon is obtained in a partly agglomerated state, it is preferredto adopt such a structure that the lower portion of the closed vesselcan be exchanged.

[0131] To cool the above silicon more effectively, the closed vessel 10is preferably provided with a cooling unit 14. As shown in FIG. 3 andFIG. 4, a liquid jacket is the most preferred in which a flow passagefor circulating a refrigerant liquid such as water, heat medium oil oralcohol from F₁₁ to F₁₂, from F₂₁ to F₂₂ or from F₃₁ to F₃₂ is formed tocool silicon.

[0132] As shown in FIG. 3 and FIG. 4, when the closed vessel 10 isconnected to an upper portion of the cylindrical vessel, the coolingunit may have a suitable jacket structure to protect the material sothat a refrigerant such as heat medium oil can be circulated. When thematerial has heat resistance, an adiabator may be used to improve a heateffect, thereby making it possible to obtain heat insulation.

[0133] As understood from the above description, the foamedpolycrystalline silicon of the present invention generates an extremelysmall amount of fine grains by crushing for the production of granularpolycrystalline silicon and is soft before crushing and extremely usefulas a silicon source in various polycrystalline silicon applicationfields.

[0134] The method of producing foamed polycrystalline silicon of thepresent invention is capable of producing foamed polycrystalline siliconwith high reproducibility and stability and is useful when it is carriedout on an industrial scale.

[0135] Further, the polycrystalline silicon production apparatus of thepresent invention is suitable for use in the above method of producingfoamed polycrystalline silicon and an industrially extremely usefulapparatus capable of continuously producing polycrystalline siliconincluding polycrystalline silicon other than the above stably at a highspeed for a long time.

EXAMPLES

[0136] The following examples are provided for the purpose of furtherillustrating the present invention but are in no way to be taken aslimiting.

[0137] The grain diameter was measured in accordance with JIS-Z8801.

Example 1

[0138] A polycrystalline silicon production apparatus similar to theapparatus shown in FIG. 3 was constructed to continuously producepolycrystalline silicon as follows.

[0139] A high-frequency induction heating coil was mounted as the heater3 around a silicon carbide cylindrical vessel 1 having an opening 2 in alower portion and an inner diameter of 25 mm and a length of 50 cm froma position 10 cm from the top to the lower end of the cylindrical vessel1. A stainless steel chlorosilane feed pipe 5 having an inner diameter10 mm and an outer diameter 17 mm and a jacket structure through which aliquid can be circulated as shown in FIG. 2 was inserted into thecylindrical vessel 1 to a height of 15 cm from the upper end of thecylindrical vessel. The closed vessel 10 had an inner diameter of 500 mmand a length of 3 m and was made from stainless steel.

[0140] The periphery of the lower end of the above cylindrical vesselhad a shape shown in FIG. 5.

[0141] Water was let pass through the cooling jacket of the chlorosilanefeed pipe to maintain the inside of the pipe at 50° C. or less, waterwas also let pass through the lower jacket of the closed vessel 10,hydrogen gas was circulated from the hydrogen feed pipe 14 at an upperportion of the cylindrical vessel 1 and the seal gas feed pipe 11 at anupper portion of the closed vessel 10 at a rate of 5 liters/min, andthen the high-frequency heater was activated to heat the cylindricalvessel 1 at 1,500° C. The inside pressure of the vessel was almostatmospheric pressure.

[0142] When trichlorosilane was supplied to the chlorosilane feed pipe 5at a rate of 10 g/min, it was observed that granular silicon dropletshaving almost the same diameter fell naturally at a rate of about 0.6g/min. In this case, the conversion of trichlorosilane was about 30%.

[0143] The silicon molten liquid was separated and dropped from theopening of the cylindrical vessel. At this point, the end of the openingin the lower portion of the cylindrical vessel got fully wet withsilicon and the surface was covered with silicon.

[0144] When operation was suspended and the inside of the apparatus wasopened and observed after a reaction was continued for 50 hours,blocking with silicon did not occur.

[0145] The above separated and dropped silicon molten liquid dropletswere naturally dropped and contacted to a cooling acceptor 9 installedat the bottom of the closed vessel 7 in 0.5 second.

[0146] The cooling acceptor 9 was cooled by filing the previouslyobtained foamed polycrystalline silicon grains therein to maintain itssurface temperature at 300° C.

[0147] The apparent density of the obtained foamed polycrystallinesilicon 10 was 1.66 g/cm³.

[0148] When the above foamed polycrystalline silicon was crushed, grainshaving no fixed shape and an average grain volume of 0.1 cc wereobtained. When each grain was broken by a hammer, a large number ofcavities formed by bubbles were observed on its broken section. When thesilicon grain was polished with diamond to observe its section, a largenumber of cavities formed by bubbles having a diameter of 0.5 to 1 mmwere existent in the center portion.

[0149] When 100 g of the above grains of the foamed polycrystallinesilicon were crushed to a maximum grain diameter of 2 mm or less by ajaw crusher to measure the grain diameter of the crushed product by theSK LASER PRO-7000 laser diffraction scattering grain size distributionmeasuring instrument (of Seishin Kogyo Co., Ltd.), the proportion offine grains passing through a sieve having an opening of 180 μm was lessthan 0.05%.

Example 2

[0150] Foamed polycrystalline silicon was obtained under the sameconditions as in Example 1 except that a silicon molten liquid wasformed from silicon tetrachloride as a source material.

[0151] When the apparent density of the solidified grain was measured,it was 2.05 g/cm³.

[0152] When the grain diameter of the crushed product obtained in thesame manner as in Example 1 was measured, the proportion of fine grainspassing through a sieve having an opening of 180 μm was 0.2%.

Example 3

[0153] A silicon molten liquid was formed by filling a graphitecylindrical vessel having a hole in a lower portion with solid siliconand heating at 1,500° C. with high frequency in a hydrogen atmosphere instead that a silicon molten liquid was formed by reactingtrichlorosilane with hydrogen. Further, after it was kept in a moltenstate for 30 minutes in the presence of hydrogen, it was pressurizedwith hydrogen from above and dropped from the hole in the lower portion.

[0154] The separated and dropped silicon molten liquid droplets weredropped naturally and contacted to a cooling acceptor 9 installed at thelower portion in 0.5 second.

[0155] The cooling acceptor 9 was cooled by filling the previouslyobtained foamed polycrystalline silicon grains therein to maintain itssurface temperature at 300° C.

[0156] When the apparent density of the solidified grain was measured,it was 2.11 g/cm3.

[0157] When the grain diameter of the crushed product obtained in thesame manner as in Example 1 was measured by the SK laser, the proportionof fine grains passing through a sieve having an opening of 180 μm was0.2%.

Comparative Example 1

[0158] Polycrystalline silicon was obtained under the same conditions asin Example 1 except that the time elapsed until the grain was contactedto the cooling acceptor was 0.05 second. Visible bubbles were notobserved in the obtained polycrystalline silicon grain. The apparentdensity of the grain was 2.25 g/cm³.

[0159] When the grain diameter of the crushed product obtained in thesame manner as in Example 1 was measured, the proportion of fine grainspassing through a sieve having an opening of 180 μm was 1%.

Comparative Example 2

[0160] A quartz plate heated at 1,350° C. with a heater installed on itslower portion was used as the cooling acceptor to gradually cool thegrain in Example 1.

[0161] Bubbles were not existent in this silicon. The apparent densityof the grain was 2.33 g/cm³.

[0162] When the grain diameter of the crushed product obtained in thesame manner as in Example 1 was measured, the proportion of fine grainshaving a diameter of 200 μm or less was 2%.

Comparative Example 3

[0163] A stainless steel chlorosilane feed pipe 5 having an innerdiameter of 10 mm and an outer diameter of 17 mm and the cooling jacketstructure 6 of Example 1 was inserted to a height of 5 cm from the topof the cylindrical vessel. Operation was carried out under the sameconditions as in Example 1.

[0164] Granular silicon could be obtained at a rate of about 0.6 g/minat the beginning of operation but after 15 hours, it became difficult tosupply trichlorosilane and seal hydrogen.

[0165] When the apparatus was opened and observed after suspension, anupper portion and a portion therearound of the inside of the cylindricalvessel 1 were almost blocked. The blocking material was silicon.

Example 4

[0166] Granular silicon was continuously obtained by constructing asilicon production apparatus shown in FIG. 4 as follows.

[0167] A high-frequency induction heating coil was mounted as the heater3 on a silicon carbide cylindrical vessel 1 having a total length of 50cm around a position 10 cm from the upper end to the lower end, in whichthe inner diameter of the insertion portion of the chlorosilane feedpipe 5 and the opening 2 was 25 mm and the inner diameter of a 20 cmcenter portion was expanded to 50 mm, and a tapered portion was formedas long as 5 cm. The stainless steel chlorosilane feed pipe 5 having ajacket structure capable of circulating a liquid and an inner diameterof 10 mm and an outer diameter of 17 mm shown in FIG. 2 was insertedinto the cylindrical vessel 1 to a height of 15 cm from the upper end.The closed vessel 10 was made from stainless steel and had an innerdiameter of 750 mm and a length of 3 m.

[0168] The periphery of the lower end of the above cylindrical vesselhad a shape shown in FIG. 6.

[0169] Water was let pass through the cooling jacket of the chlorosilanefeed pipe to maintain the inside of the pipe at 50° C. or less, waterwas also let pass through the lower jacket of the closed vessel,hydrogen gas was circulated from the hydrogen feed pipe 14 at an upperportion of the cylindrical vessel 1 and the seal gas feed pipe 21 at anupper portion of the closed vessel 10 at a rate of 5 liters/min, andthen the high-frequency heater was activated to heat the cylindricalvessel 1 at 1,500° C. The inside pressure of the vessel was almostatmospheric pressure.

[0170] When trichlorosilane was supplied to the chlorosilane feed pipe 5at a rate of 10 g/min, it was observed that granular silicon dropletshaving almost the same diameter fell naturally at a rate of about 1g/min. In this case, the conversion of trichlorosilane was about 50%.

[0171] When operation was suspended and the inside of the apparatus wasopened and observed after a reaction was continued for 50 hours,blocking with silicon did not occur.

1. Foamed polycrystalline silicon which has bubbles therein and anapparent density of 2.20 g/cm³ or less.
 2. The foamed polycrystallinesilicon of claim 1 which is in the form of an assembly of independentgrains or an agglomerate of independent grains.
 3. The foamedpolycrystalline silicon of claim 2, wherein the assembly of independentgrains contains independent grains each having a weight of 0.2 to 2 g inan amount of 50 g or more based on 100 g.
 4. The foamed polycrystallinesilicon of claim 2, wherein the assembly of independent grains is formedby breaking up the agglomeration of an agglomerate of independentgrains.
 5. The foamed polycrystalline silicon of claim 1, wherein aplurality of independent bubbles are contained and are existent in acenter portion of a grain.
 6. A crushed product of the foamedpolycrystalline silicon of claim
 1. 7. Th e crushed product of claim 6which has an average grain diameter of more than 200 μm and 5 mm orless.
 8. A method of producing foamed polycrystalline silicon comprisingnaturally dropping droplets of silicon containing hydrogen which hasbeen molten in the presence of hydrogen in 0.2 to 3 seconds and coolingthe droplets until hydrogen bubbles are locked up in the droplets. 9.The method of claim 8, wherein natural dropping is carried out for 0.2to 2 seconds.
 10. The method of claim 8, wherein a silicon depositionreaction between hydrogen and a chlorosilane and a reaction for meltingthe deposited silicon in the presence of hydrogen are carried outsimultaneously to prepare silicon droplets containing the hydrogen. 11.A polycrystalline silicon production apparatus comprising: (a) acylindrical vessel having an opening which is a silicon take-out port atthe lower end; (b) a heater for heating the inner wall from the lowerend to a desired height of the cylindrical vessel at a temperature equalto or higher than the melting point of silicon; (c) a chlorosilane feedpipe which is composed of an inner pipe having a smaller outer diameterthan the inner diameter of the cylindrical vessel and constituted suchthat one opening of the inner pipe faces down in a space surrounded bythe inner wall heated at a temperature equal to or higher than themelting point of silicon; and (d) a first seal gas feed pipe forsupplying seal gas into a space defined by the inner wall of thecylindrical vessel and the outer wall of the chlorosilane feed pipe. 12.The apparatus of claim 11 which further comprises (e) a hydrogen gasfeed pipe for supplying hydrogen gas into the above cylindrical vessel.13. The apparatus of claim 11, wherein a cooling acceptor for receivingdroplets falling from the lower end of the cylindrical vessel isdisposed in a lower portion of the cylindrical vessel with a spacetherebetween.
 14. The polycrystalline silicon production apparatus ofany one of claims 11 to 13 further comprising a closed vessel whichcovers at least a lower end portion of the cylindrical vessel, forms aspace in the lower portion of the cylindrical vessel and is providedwith an exhaust gas discharge pipe, and a second seal gas feed pipe forsupplying seal gas into a space defined by the outer wall of thecylindrical vessel and the inner wall of the closed vessel.